Method of making superalloys

ABSTRACT

A PROCESS FOR MAKING A SUPERALLOY HAVING IMPROVED PROPERTIES WHEREBY A MOTEN ALLOY IS INITIALLY MICROCAST IN A NONOXIDIZING, SUBSTANTIALLY MOISTURE-FREE ATOMOSPHERE, FORMING A METALLIC POWDER OF A CONTROLLED PARTICLE SIZE AND SHAPE AND HAVING AN OXYGEN CONTENT OF LESS THAN ABOUT 100 P.P.M., AND THEREAFTER CONFINING THE POWDER IN A DEFORMABLE METAL CONTAINER WHILE IN A PROTECTED ATMOSPHERE AND SIMULTANEOUSLY COMPACTING AND FORGING THE POWDER AT AN ELEVATED TEMPERATURE AND AT A CONTROLLED RATE, FORMING A SUBSTANTIALLY DENSE SOLID MASS CHARACTERIZED BY ITS SUBSTANTIALLY UNIFORM FINE-SIZED WROUGHT-TYPE GRAIN STRUCTURE.

June 20, 1972 J. w.sMYTHE EI'AL 3,671,230

METHOD 0F MAKING SUPERALLOYS mea Feb. 19, 1969 United States Patent Ollice 3,671,230 Patented June 20, 1972 3,671,230 METHOD OF MAKING SUPERALLOYS John W. Smythe and Philip I. Karp, Ann Arbor, Mich., assgnors to Federal-Mogul Corporation Filed Feb. 19, 1969, Ser. No. 800,541

Int. Cl. B22f 1/00 U.S. Cl. 75-213 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION A variety of so-called superalloys have been developed which are primarily nickel, iron and/ or cobalt-based alloys which, due to their excellent oxidation resistance and mechanical properties at elevated temperatures, that is, usually above l400 F., are receiving widespread acceptance in aerospace technology and in components for gas turbines subjected to high stresses at elevated temperatures. The development of new versions of such superalloys has been stimulated by the increasing demand for improved performance and durability of structural components subjected to elevated temperatures, as well as enabling apparatuses, such as gas turbines, to operate at higher temperatures with a corresponding increase in their operating efficiency. The attainment of improved elevated temperature properties of such superalloys, including improvements in their tensile strength, creep resistance, thermal fatigue and corrosion resistance, has been attained by a relatively close control of a complex alloy chemistry involving the use of a large number of different alloying constituents which has resulted in a corresponding increased diiiiculty in the fabrication of such superalloys, In view of the foregoing, such superalloys are conventionally cast to shape but the complex chemistry thereof results in castings having a nonuniform grain structure, and a lack of homogeneity which is primarily caused by the segregation of massive carbides and intermetallic phases. The resultant castings are themselves dilicultA to post-work and the components made therefrom possess physical properties less than optimum.

In order to overcome the problems and difficulties associated with cast billets of superalloys, it has heretofore been proposed to form such billets employing powder metallurgical techniques by which the superalloy is irlst reduced to a nely-particulated state and thereafter is consolidated and heated to an elevated temperature and hot-pressed into a relatively dense mass. While such hotpressed billets of superalloy powders are devoid of the conventional voids, blow holes or pockets associated with cast billets of these same alloys, the microstructure of such hot-pressed billets is characterized as containing an unrecrystallized relatively brittle grain boundary corresponding substantially to the boundaries of the individual particles of which the billet is comprised. The presence of such brittle grain boundaries prevents the attainment of optimum highvtemperature physical properties of components made therefrom. Such hotepressed alloys do not permit optimum utilization of the hardening agents incorporated in the alloy and controlled grain growth of the Ialloy during subsequent heat treating operations to achieve improved physical characteristics. Attempts to reduce the deficiencies of such hot-pressed powder metallurgical billets by subsequent forging operations performed on the billet have not been entirely satisfactory in achieving an improvement in the resultant high temperature physical properties of the material.

The present method overcomes the problems and diiculties of the techniques heretofore known and proposed by employing powder metallurgical techniques whereby the resultant .superalloy powder and iinal billet are of a controlled maximum combined oxygen content and wherein the unique combination of controlled steps effects the formation of a billet having a relatively uniform iine` sized wrought-type grain structure providing physical properties which heretofore were unattainable in alloys of similar chemistry. A particular advantage of the method comprising the present invention is the uniformity of microstructure and physical properties of successive billets, as well as uniformity of these same properties within each individual billet.

SUMMARY OF THE INVENTION The benefits and advantages of the present invention are achieved in accordance with a process by which a superalloy of the desired chemistry i s initially microcast, such as by atomization, in a substantially inert atmosphere, forming spherical powder particles which have an oxygen content of less than about 100 p.p.m. (parts per million) and preferably less than about p.p.m. The metal powder thus formed is screened to provide a powder having particles less than mesh, which is subsequently transferred and confined within a deformable metal container in a manner so as to minimize exposure of the powder to oxygen to assure that the oxygen content thereof remains below the aforementioned maximum content. The container thereafter is evacuated, sealed and heated to an elevated temperature, such as from about 2000 F. to about 2500 F., whereafter a force is applied to the container and to the powder therein in a direction and at a controlled rate effecting a concurrent compaction and forging of the powder and a coalescence of the particles, forming a substantially dense solid body characterized as having a uniform fine-sized grain structure typical of wrought materials and wherein the oxygen content of the resultant body is less than 100 p.p.m. The container is thereafter removed from the exterior of the solid metal body which can be directly employed for the fabrication of structural components. The resultant solid body is further characterized as possessing reproducible high temperature physical properties which are substantially superior to the physical properties of alloys of similar chemistry made by alternative techniques of the types heretofore known.

Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a flow diagram illustrating the sequence of steps of the method comprising the present invention;

FIG. 2 is a transverse vertical sectional view through a deformable metal container filled with a superalloy powder;

FIG. 3 s a fragmentary transverse vertical sectional view through a typical press in which the preheated filled container is compacted and forged; and

FIG. 4 is a photomicrograph of the grain structure of the superalloy produced in accordance with the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the drawing, the sequence of the steps in accordance with the preferred practice of the method comprising the present invention is schematically illustrated in FIG. 1. As shown, an alloy is initially molten and thereafter microcast, such as by atomization, forming a metallic powder in which each of the individual particles possess substantially the same alloy chemistry. The powder thereafter is screened so as to extract particles of the requiste size range, which thereafter are placed in a deformable metal container and sealed therein prior to pre-heating. After the container and its contents have attained the desired uniform elevated temperature, the container and its contents are subjected to a concurrent compaction and forging step in which a coalescence and desication of the powder is accomplished forming a dense billet of uniform grain structure. After cooling, the container encircling the periphery of the billet is stripped leaving a billet of the superalloy having a wrought-type grain structure which can be directly used without further treatment for the fabrication of structural components.

The process of the present invention is particularly applicable and provides important benets in connection with the formation of billets of so-called Superalloys which are primarily of a nickel, iron or cobalt base incorporating a large number and variety of additional alloying constituents. Superalloys are conventionally characterized by high oxidation resistance and excellent mechanical properties, including, for example, tensile strength, fatigue and thermal shock resistance, when subjected to temperatures usually above 1400 F. and frequently as high or higher than 1800" F. Elevated temperatures within the foregoing ranges are frequently encountered in sections of gas turbines, such as turbine blades, stator vanes, etc. Alloy compositions of typical Superalloys to which the present invention is applicable are listed in Table 1. It will be appreciated that the enumerated Superalloys are illustrative of commercially available materials and are not intended as being restrictive of other alloy variations to which the practice of the present invention is also applicable.

4 described in U.S. Pat. No. 3,253,783, which is assigned to the same assignee as the present invention. The atomization and collection of the microcast alloy is achieved under conditions whereby oxygen or oxygen-containing substances, such as water, are excluded whereby the resultant metallic powder has a combined oxygen content of less than 100 p.p.m. and preferably less than about 80 p.p.m. The control of the oxygen content of the powder below 100 p.p.m. is an important feature of the present invention and is necessary for achieving a resultant dense billet possessing the improved mechanical properties. The precautions required in the atomization and subsequent handling of the superalloy are dependent upon the presence and amounts of certain alloying elements, such as aluminum and titanium, for example, which are susceptible to oxidation particularly at elevated temperatures.

The exclusion of appreciable amounts of oxygen from contact with the alloy during the atomization process can be achieved by any one of a variety of techniques and preferably is achieved by surroundnig the melt and yflooding the collection chamber with a substantially dry inert or non-oxidizing atmosphere such as one or a mixture of inert gases. For this purpose, commercially available argon containing minimal contaminants and moisture can be satisfactorily employed as the substantially nonoxidizing moisture-free atmosphere. The removal of oxygen from contact `with the alloy during its melting and its subsequent microcasting can be conveniently achieved by preliminarily evacuating the chambers in which the alloy is confined and thereafter back-flooding with the inert gas.

The atomization or microcasting of the alloy is performed in a manner so as to form a metallic powder in which the particles are of a generally spherical configuration and wherein each particle has a similar alloy chemistry. The resultant powder is thereafter recovered and is subjected to a screening operation to extract particles suitable for forming the superalloy billet which range in particle size of less than about 100 mesh (U.S. Standard Sieve Size), and preferably less than about mesh. The minimum particle size may be as small as one micron but generally, particles of about l0 microns or larger are preferred. In accordance with the preferred practice, the particles are of sizes randomly distributed over the total acceptable size range, that is, preferably from about 100 mesh to about 10 microns, to approach maximum packing density.

TABLE 1 Nominal compositions of sonic nickel-base superalloys (percent by weight) Alloy C Cr Al Ti Mo W Co Cb B Zr Other Ni Nimonic 0.12 20 0.5 Balance. Nimonic A. 20 1. 5 2. 4 Do. Nimonc 20 1. 6 2. 4 Do. Nmonic 20 2.0 3.0 Do. Nimonic 100. 11 5.0 1. 3 Do. Waspaloy 0. 08 l!) l. 3 3. 0 Do. Udimct 700. l5 4. 3 3. 5 Do. Rene 41 1Q 1.5 3.1 Do. :iN- (Cast) l0 5. 5 5. 0 Do. MAR-H200 (c t) 9. 0 5.0 2.0 Do. 13-1900 (east) 8. 0 6. 0 1. 0 6. 0 Do. INCO-713 (Cast) 13. 0 6. 0 0. T5 4. 5 D0. 1\I-252 19, 0 1.0 2. 5 9. 8 Do.

Cb plus Ta. b Max.

As will be noted in the foregoing table, the super-type alloys typically are nickel-base containing substantial proportions of cobalt and chromium together with lesser amounts of additional alloying agents for imparting hardening and improved high temperature strength by intermetallic phase strengthening, solid solution hardening and carbide strengthening.

In accordance with the practice of the present invention, a melt is prepared of the superalloy of the desired alloy chemistry and is thereafter microcast forming a metallic powder in which each of the particles are of substantially the same composition. The microcasting of the molten alloy can be conveniently achieved by an atomization process employing a nozzle arrangement such as The screening of the microcast superalloy is also preferably performed under conditions in which oxygen is eX- eluded from contact with the powder particles so as to prevent any appreciable oxidation attack thereof to avoid the combined oxygen content thereof from exceeding a level of 100 p.p.m., and preferably below a level of 80 p.p.m. This can conveniently be achieved by performing the screening operation in a sealed chamber from which air has been evacuated and which subsequently s backooded with a suitable inert gas, such as argon, for example. The sealed chamber is provided with suitable manipulative means for handling the powder to effect a classification thereof into the appropriate sizes for subsequent transfer to a deformable metal container.

The superalloy powder of the desired alloy chemistry and particle size range is thereafter transferred to a deformable metal container, such as the container 10 illustrated in FIG. 2. The container 10, as illustrated, is of a right cylindrical configuration consisting of a cylindrical side -wall 12 and a pair of circular end walls 14. The upper end wall is provided with a tubular stem 16 disposed in communication with the interior of the container by which the container is filled with the superalloy powder. The connection of the stem 16 and the junctures of the end walls and the side walls are constructed so as to provide an airtight high-strength joint which can conveniently be achieved such as by welding. The container itself may comprise any suitable metal having sufficient ductility to enable its deformation at an elevated temperature without rupture to assure the maintenance of the air-tight integrity of the interior thereof. For this purpose, any ductile metal which is compatible with the superalloy powder contained therein can be satisfactorily employed for this purpose such as any one of the conventional so-called stainless steels such as type 304 or a 1010 mild steel.

The container itself is of a controlled configuration in order to provide the requisite compaction and forging of the powder particles forming a resultant dense superalloy billet during the subsequent hot compacting operation. In order to achieve a satisfactory grain structure, it haS been found necessary to employ containers having a height corresponding to the distance between the end walls to a diameter ratio ranging from about 0.5:1 to about 2:1, and preferably a height to diameter ratio of about 1:1. The application of the force effecting deformation of the container is in a direction substantially parallel to the axis of its annular side wall 12 in a manner subsequently to be described.

The filling of the container similarly is achieved under conditions so as to minimize oxidation attack of the metallic powder particles and may conveniently be achieved by employing the same chamber in which a screening of the powder particles is performed. In either event, the interior of the container is first evacuated to remove substantially all of the air therefrom and may be filled in that condition or, alternatively, can be back-flooded with a suitable inert gas, such as argon for example. In order to assure the maximum compaction of the loose powder particles within the interior of the container, it is preferred to vibrate the container at sonic or supersonic frequencies achieving a packing thereof at a magnitude usually ranging from about 60 %to 70% of a theoretical 100% density depending on the specific particle size and size distribution of the powder.

After the container has been substantially completely filled with powder, the inert gas is evacuated from the container and the tubular stem is closed, such as by crimping or pinching the outer projecting end indicated at 18 in FIG. 2, and the crimped portion is thereafter welded along its outer edge to further insure the mainenance of air-tight integrity.

The sealed container 10, as shown in FIG. 2, containing the metallic powder 20 sealed therein can thereafter be readily handled without fear of exposure of the powder to oxidation attack, assuring the maintenance of a combined oxygen contact less than the 100 p.p.m. level. In accordance with the next processing step, the filled and sealed container is subjected to a preheating operation in which the container and its contents are heated to an elevated temperature approaching the solids or just below the incipient melting point of the powder particles which usually ranges from about 2000 F. to about 2500 F. preliminary to the compacting operation. The heating of the container and its contents can be achieved in any one of a Variety of suitable furnaces and is continued for a period of time sufficient to assure substantial uniformity in temperature of the powder charge. The container thereafter is transferred to a press, such as illustrated in FIG. 3, and is positioned on a lower die shoe or base 22, having an annular retaining Wall 24 therearound. Thereafter, the ram or punch 26 is lowered at a controlled rate effecting an axial compaction and lateral deformation of the container and the powder therein from the configuration as shown in phantom in FIG. 3 to a flattened disk form 28 as shown in solid lines. During the compaction and forging of the powder, the periphery of the deformed container contacts the inner surface of the annular wall 24, thereby assuring proper compaction and densification of the peripheral portions of the alloy in the disk 28.

In order to achieve a resultant billet of the superalloy having a substantially uniform wrought-type grain structure, it is necessary that the rate of compaction and forging of the preheated powder in combination with the height to diameter ratio of the container be controlled within a preselected range. For this purpose, the permissible rate of axial compaction of the container has been found to be less than about 200 inches per minuteto a minimum rate of about ten inches per minute. Preferably, the rate of axial compaction for most containers, particularly those of a height to diameter ratio of about 1:1, is within the range from about 60 to about 80 inches per minute. This relatively slow rate of axial compaction has been found necessary to achieve the desired metallurgical structure and attendant superior mechanical properties of the resultant superalloy billet. Rates above about 150 inches per minute have been found to result in a nonuniform grain structure, as well as the presence of grain boundaries corresponding to the original powder particles indicating incomplete coalescence of the individual particle while rates below the minimum rate of about ten inches per minute similarly result in inferior metallurgical grain structures. It is for this reason that the rate of compaction must be carefully controlled Within the aforementioned permissible range which constitutes an important feature of the present invention.

Referring again to FIG. 3, it will be noted during the downward movement of the ram or punch 26, the container 10 and the powder contents thereof are axially compressed and thereafter are radially extruded eventuating into the flattened disk configuration indicated at 28. During this compaction and concurrent extrusion operation of the preheated powder, a densification thereof occurs approaching a theoretical density and a coalescence of the individual particles wherein the resultant wrought-type grain structure is substantially devoid of grain boundaries corresponding to the original particle boundaries. The resultant grain structure is illustrated in the photornicrograph of FIG. 4 taken at a magnification of 100 times. The grain structure as shown is typical of wrought structure which enables the resultant superalloy billet to be used in that condition for the fabrication of components which possess superior oxidation resistance and mechanical properties at elevated ternperatures.

At the completion of the compaction and extrusion operation resulting in the formation of the flattened disk 28 containing as a core the substantially dense solid superalloy billet, the upper ram 26 is retracted and the disk removed. After cooling, the deformed metal container enveloping the periphery of the billet is removed, such as by machining or other suitable means, to expose the resultant billet which can be further trimmed along its peripheral portions into a disk suitable for fabricating components requiring excellent mechanical properties at elevated temperatures.

The superiority of the superalloys made in accordance with the practice of the present invention in comparison to superalloys of similar composition made in accordance with prior art practices is evidenced by the data as set forth in Table 2. The material designated Sample A in Table 2 corresponds to a superalloy having a cornposition of the Udimet 700 material as set forth in Table 1 which was made in accordance with the practice of the present invention having a grain structure corresponding to that as shown in FIG. 4. The Udimet 700 material was microcast by atomization and particles of a size ranging from 100 mesh to 10 microns were recovered and placed 5 in a deformable container which was evacuated, such as shown in FIG. 2, having a H:D ratio of 1:1. The powder and container were preheated to a temperature of about 2200 F. and compacted at a ram rate of 80 inches per 8 confined said powder to an elevated temperature of from about 2000" F. to about 2500 F., applying an axial force to the heated said container in a direction parallel to its axis at a rate of about 10 up to about 150 inches per minute effecting an axial and substantial radial deformation of said container and a compaction and lateral radial outward forging and coalescence of said powder therein into a substantially dense solid disc having a substantially uniform wrought grain structure, and thereafter removing minute. The combined oxygen content of the resultant 10 the container from the periphery of said solid disc. billet was about 50 p.p.m. The Sample B specimen of 2. The method as dened in claim 1, wherein said Table 2 corresponds to this same alloy chemistry which microcasting is achieved by an atomization of the molten was made in accordance with the prior art technique of said metal into substantially spherical particles. preliminarily casting the alloy, forming a cast ingot which 3. The method as defined in claim 1, wherein the subsequently was wrought by hot forging. oxygen content of said powder in said container is less than about 80 p.p.m.

TABLE 2 Yield Strength, Ultimate Percent Life (hrs.) 0.2% p.s.i. strength, p.s.i. elongation 1,400" F. 85,000 ps1. Sample 70 I". 1,400 F. 70 F. 1,400o F. 70 F. 1,400 F. stress A 100,000 143, 000 215, 000 170,000 16 20 4s B 140.000 125,000 195, 000 150,000 16 2o 30 It will be noted that the alloy corresponding to Sample A made in accordance with the practice of the present invention possesses a significant superiority in its low and high temperature yield strength and ultimate strength, as well as in its useful life under high stress at elevated temperatures. Of equal importance and a further advantage of the present method is the uniformity of the grain structure and the mechanical properties of the superalloy billet made by the present method throughout its entire volume, as well as the uniformity of such properties between throughout its entire volume, as well as the uniformity of such properties between successive billets. In view of the foregoing, the flattened disk 28, after removal of the enveloping deformed metal container, can readily be employed for fabricating a turbine hub, for example, in the as is condition orwith minimal additional forging as may be desired.

While the description of the preferred embodiments of the present invention are well calculated to achieve the advantages and benefits hereinabove stated, it will be appreciated that the process is susceptible to variation and change without departing from the spirit of the invention.

What is claimed is:

1. The method of making a disc shaped metallic mass of a superalloy having a wrought grain structure which comprises the steps of microcasting a molten mass of a metal in a substantially nonoxidizing moisture-free atmosphere forming a metallic powder having an oxygen content less than about 100 p.p.m. and an average particle size distributed over a range of from less than about 60 mesh to about 10 microns, confining said metallic powder in an evacuated deformable metal container in a manner to prevent the oxygen content of said powder from exceeding about 10G p.p.m., said container being of a substantially right cylindrical configuration and having a height to diameter ratio of from about 2:1 to about 0.5 :1, heating the 4. The method as defined in claim 1, in which said atmosphere in which the microcasting is performed is composed of argon.

5. The method as dened in claim 1, in which said container has a height to diameter ratio of about 1:1.

6. The method as described in claim 1, in which the rate of axial deformation of said container ranges from about 60 to about 80 inches per minute.

References Cited UNITED STATES PATENTS 2,966,735 1/1961 Towner et al. 75-226 X 2,967,351 1/1961 Roberts et al. 29-420.5 3,341,325 9/1967 Cloran 75--226 X 3,343,998 9/1967 Ablkowitz 75-214 X 3,384,481 5/1968 Broverman 75-226 X 3,459,546 8/1969 vLambert 75--214 X 2,701,775 2/1955 Brennan 18-2.5 X 2,587,614 3/1952 Golwynne 18--2.5 X 2,638,627 5/1953 Golwynne 18--2.5 X

FOREIGN PATENTS 719,047 11/ 1954 Great Britain.

OTHER REFERENCES Orrell, Beryllium Powder lForging, New Methods for the Consolidation of Metal Powders, Plenum Press, New York, 1967, pp. 204-207.

BENJAMIN R. PADGETT, Primary Examiner R. L. TATE, Assistant Examiner U.S. C1. X.R. 

